In the chemical process and petroleum refining industries, maintenance and often capital improvements of process plants occur on a scheduled basis, either annually, biannually, or less often, if possible, at scheduled times when the operating unit is completely shut down and every aspect of its operation inspected and maintained. Operating units are made up of various types of vessels; i.e., tanks, heat exchangers, distillation columns, reactors and the like in fluid communication with each other. Characteristically, in refining and petrochemical operations, a feed, such as crude oil, is introduced to one end of the plant, with hydrocarbon product stream being removed at the other end, to storage or transporting facilities. Of course, any similar process flow-stream equipment is also cleaned. This massive maintenance effort on each unit is called a turnaround and maximum speed in returning the unit to operation is essential to the plant economics, since the main cost of such an operation, even though itself expensive, is the time during which this operating unit is not producing refined product which can be sold. The speed at which the jobs are done are, of course, tempered with the necessity that safety of the workers be maintained throughout the operation.
During a turnaround, an operating process cannot merely be shut down and drained to allow entry of maintenance and repair crews. The process equipment must first be rendered safe and decontaminated of any residual material in it which may be harmful, even fatal, to the workers who must enter to inspect and maintain, if not refurbish, the internal structure of the equipment. In maintaining and repairing the processing equipment, it has normally been necessary to clean and decontaminate single, or groups of vessels, before entry for maintenance and repair. The present practice, depending upon the equipment, uses a light hydrocarbon solvent to initially remove heavy oils and tars, followed by injection of steam for a period of time until monitoring devices indicate that no dangerous gases remain which present an explosive hazard to workers who must work in this environment. Process equipment was also washed with water to remove contaminants where applicable, and often, both steaming and water washing is involved in the degassing of a vessel to make it possible for humans to safely enter to inspect and repair. These steps often take many hours, even days, to accomplish--days of lost production. The often ineffective steam purging step is eliminated; after the solvent step, also. It is desirable of course to effectively clean and decontaminate the entire process flow-path at one time.
Following degassing, prior art methods of cleaning processing systems typically relied upon chemical reactions wherein strong or weak, inorganic or organic acids, bases, or chelating agents (such as HF, HCl, H.sub.2 SO.sub.4, citric, acetic, and hydroacetic acids or EDTA, DTPA, and similar chelating agents) were pumped through the process vessels and equipment to remove inorganic metallic oxide scales, corrosion products and hydrocarbon deposits. Unfortunately, corrosive chemicals aggressively attack not only the scales and contaminants, but also the materials of construction, as well as the contamination and could not be pumped through existing process pumps or lines because they can cause damage to the mechanical seals of the pumps. It was necessary to incorporate independent, smaller pumps and smaller lines to perform the cleaning process, resulting in slower, less efficient cleaning. And of course, an acid cleaning required a flushing step, a neutralizing step, and another flushing step--all requiring the handling of chemicals difficult to dispose of and causing further time delays. Less reactive chemicals did not fully clean the equipment either. Neither treatment processes accomplish vapor phase cleaning and remove dangerous volatile organic contamination.
Of particular importance to the safety of workers is the removal of benzene and other volatile organic carbon components from the process vessels. It has been long recognized that chronic exposure by humans to benzene at high levels in the chemical and petrochemical work place leads to bone marrow depression, aplastic anemia, and leukemia. Although absorption of benzene across the skin as a vapor or in aqueous media can occur, benzene toxicity in process systems is most frequently caused by inhalation of benzene that has managed to escape removal. Present government safety standards for eight hour work days are set at 1.0 ppm (average) benzene. The National Institute For Occupational Safety And Health (NIOSH) has recommended (1989) an occupational long term exposure limit in air of 0.1 ppm benzene. That is not good enough, since workmen employed by turn-around service companies are continually exposed to the interior of process vessels. Worker safety regulations now limit such exposure. One major goal of the present invention, therefore, is to provide a process for process equipment decontamination which exceeds this standard--in fact, which approaches, if not meets, 0 ppm of benzene. Benzene is often found to be trapped beneath scale or other contaminants anywhere along the process flow stream--in piping, in valves, and in pumps, as well as towers, reactors, tanks and heat exchangers--only to seep out from the interstices of contamination deposits at a later time when cleaning had been thought to be completed or to collect in head space of the vessel. Until now, there was no process for decontaminating the entire process flow-path with one unitary process.
Even though there are many conflicting problems which converge on the process equipment, still, the overriding requirements are speed with safety, and the practice of this invention accomplishes these while providing avenues for a more effective protection of the environment. This invention allows almost pristine cleaning of process equipment--from the crude oil feed pump to product storage tank--without even opening them, in many cases until safely decontaminated of dangerous substances, particularly such as benzene.